Structures such as aircraft airframes, ships' hulls, and bridges require regular inspections to check for damage. Inspections are currently usually performed manually according to a schedule. These scheduled inspections are precautionary, and, often, no damage is found. Such inspections are very time consuming and thus costly, since the structure will be out of use whilst the inspection is carried out. However, they are necessary since the consequences of structural failure can be catastrophic.
A number of damage or defect sensor systems are currently being developed. These systems aim to eliminate costly manual technology by enabling structures to perform ‘self-inspection’ using automated networks of sensors. Such self-inspection systems, if available, would allow the owners and operators of structures to benefit from lower operating costs and less frequent disruptions to use, since the structure would only be out of use if actual maintenance, to repair actual damage, were necessary. Owners and operators would also benefit from lower risk of structural failure, and therefore enhanced safety, since self-inspection systems would enable structures to be continuously monitored throughout their lives, and thus any defects in or damage to the structure would be detected sooner.
Many current sensor concepts are described in “Proceedings of the 5th International Workshop on Structural Health Monitoring”, Stanford University, Stanford, Calif., September 2005, edited by Fu-Kuo Chang. Current techniques use powered, discrete sensors that actively probe structures using ultrasound, or use highly sensitive ultrasonic microphones that ‘listen’ for cracks. In currently known sensor systems a compromise must be reached between a number of conflicting factors, such as the complexity of the sensor devices, the number needed to cover a given structure, the sensitivity of the sensor devices, the size and weight of sensor installations, and the overall cost of the sensor system. For example, if it is desired to monitor a ship's bulk for damage using prior-known discrete sensors, it is necessary to use a large number of sensors in order to reliably monitor the entire hull with an appropriate degree of sensitivity. However, the cost, complexity and weight of the system increases with the number of sensors used. Furthermore, individual connections must be made to each sensor. The reliability of any electrical system decreases as the number of electrical connections required increases. The production time for the structure also increases as the number of electrical connections increases, thereby also increasing manufacture costs. For example, fitting discrete strain gauges to a modern military aircraft can add several weeks to the production time. Such sensing systems, if used for damage detection are therefore not readily scalable.
One example of a prior-known system for damage detection is that described by Mark W. Lin, Ayo O. Abatan, and Musa B Danjaji in their paper “Electrical time domain reflectometry sensing cables as distributed stress/strain sensors in smart material systems”, Proceedings of SPIE Volume 3042, pages 33 to 41, June 1997. Lin et al. disclose the use of electrical time domain reflectometry (ETDR) for health monitoring applications of civil engineering structures. The ETDR sensing method disclosed by Lin et al. requires the use of a coaxial cable embedded in the structure. The cable is deformed by the stresses and strains to which the structure is exposed, and these deformations of the cable result in impedance changes which will partially reflect a signal propagating in the cable. The magnitude of the reflection is related to the degree of deformation of the cable, and so, by monitoring the cable, the stresses and strains to which the structure is exposed can be monitored. Unfortunately, it is not clear how the deformation of the cable relates to the stresses and strains to which the structure is subjected, and nor is it clear how an instantaneous measure of stresses and strains in the structure can be used to assess the level of damage to the structure.